In recent years, with a view to global environmental protection and improvements in machinery efficiency, investigations have been made into the use of supercritical refrigerants such as carbon dioxide, etc., as air conditioner refrigerants in which a high-pressure side can be operated in a gas-liquid two-phase region or a supercritical region depending on such conditions as thermal load. Examples of air conditioners that use refrigerants that can be operated when high-pressure pressure is greater than or equal to critical pressure include supercritical vapor compression cycle operating methods and operating devices such as that described in Patent Literature 1, for example.
In a supercritical vapor compression cycle operating method such as that described in Patent Literature 1, a supercritical vapor compression cycle apparatus includes a compressor, a high-pressure heat exchanger (radiator), a throttling means, and an evaporator, and characteristics of inflow temperature of fluid that flows into the radiator and exchanges heat with the refrigerant, and a high-side pressure at which a coefficient of performance of the vapor compression cycle is maximized are found in advance. Thus, the inflow temperature of the fluid (an indoor temperature, for example) is measured as a control variable for the vapor compression cycle, and if this measured value has not reached a target value and capacity is required from the vapor compression cycle, a degree of opening of the throttling means is adjusted so as to operate at a high-side pressure at which the coefficient of performance is maximized based on the characteristics of the inflow temperature of the fluid and the high-side pressure that have been found in advance. If, on the other hand, capacity is not required from the vapor compression cycle, the degree of opening of the throttling means is adjusted so as to lower the high-side pressure to a preset minimum high-side pressure and reduce capacity, and if operation at this minimum high-side pressure continues for a preset specific amount of time, the compressor is stopped, and on-off control of the compressor is performed manually.    Patent Literature 1: Japanese Patent Laid-Open No. HEI 11-211251 (Gazette)